The art is so good! The swimming animation is smooooth and the kelp and the parallax effect is just perfect. I really like difficult platformers, but this was just a bit too much. Couldn't get past the spikes part for the life of me. Either died to the spikes if I went fast, or died to oxygen/time if I was slow and careful. I would maybe balance those two things a little more in the level design (sometimes fast with few dangers but also few oxygen tanks, then areas with lots of dangers but also lots of oxygen). The controls felt good, especially when you hit a streak of good boosts.
Is the oxygen draining somehow tied to framerate? It ran out really quickly.